In 3GPP E-UTRA Rel-10 (LTE-Advanced), multiple component carriers can be aggregated in the uplink and downlink, respectively. For example, to provide high data rates, a User Equipment (UE) can receive simultaneous transmissions on multiple downlink component carriers. Sometimes the notion of a cell is used instead of component carrier, and aggregation could equivalently be for cells. In an aggregation case the UE may be configured with multiple serving cells. A cell may provide both an uplink and a downlink direction of communication and may thus comprise both an uplink and downlink component carrier. Hence, the definition of a cell in mentioned standard's context is not limited to a geographical location and one base station (eNodeB) may facilitate multiple serving cells. A UE can be assumed to have a Primary serving cell (PCell) as well as one or several Secondary serving cells (SCells). A person skilled in the art could equivalently use the terminology of cells instead of component carriers in relation to carrier aggregation.
At the transmitter, data is encoded, modulated and arranged in a transport block. Each component carrier is individually configured for either spatial multiplexing or no spatial multiplexing. For one UE, when spatial multiplexing is not configured, a component carrier is used for transmission of 1 transport block, when spatial multiplexing is configured, i.e. MIMO transmission, up to 2 transport blocks are transmitted on the component carriers. If the detection of a transport block is successful, the UE sends an ACK message on the uplink and if the detection is unsuccessful, a NACK message is sent. Thus, with carrier aggregation, multiple ACK or NACK messages (also referred to as HARQ-ACK bits or states) need to be transmitted from the UE in response to the transmitted transport blocks over different downlink component carriers.
Before processing the transport block for dynamically scheduled data transmissions a Physical Downlink Control Channel (PDCCH) first needs to be detected which contains the downlink assignment information needed to receive the data channel and to decode the transport block. If the UE did not correctly receive the control channel, the UE is not aware that it is expected to receive any data and it does not send any feedback; neither ACK nor NACK in the uplink. This is referred to as discontinuous transmission (DTX). DTX is thus also expected if the eNodeB did not transmit any PDCCH. Data can also be received without an associated PDCCH, i.e. semi-persistent scheduling (SPS). For SPS, resources for the data channel are assigned to the UE for longer periods.
The eNodeB knows when to expect a NACK or ACK and upon DTX detection, the eNodeB would have to initiate a re-transmission, if it had transmitted the PDCCH. One PDCCH transmission contains the assignments for both transport blocks in a MIMO transmission. If the PDCCH is missed, both assignments are lost. Hence, DTX applies to both transport blocks simultaneously for MIMO transmissions.
In addition to missing a downlink assignment, the ACK/NACK signaling in the uplink may be erroneous, e.g. a transmitted ACK may be received as a NACK, or a transmitted NACK may be received as an ACK. A NACK-to-ACK error may introduce HARQ buffer corruption due erroneous combination of several transmissions since the UE may expect a retransmission while the eNodeB schedules a new packet. An ACK-to-NACK error causes inefficient system operation due to unnecessary retransmissions which the UE is not expecting. It is therefore important to provide robust ACK/NACK signaling.
Channel selection is one method that is capable for transmission of multiple ACK and NACK bits. The method assumes that a set of channels is reserved for the UE. The HARQ-ACK information feedback transmission is performed on the selected channel by QPSK modulated sequences and the feedback information is encoded by both the selection of the channel in the form of sequence and the QPSK constellation point. This is referred to as PUCCH format 1b with channel selection.
The channel selection refers to the selection of the sequence, and several orthogonal sequences can be transmitted on the same frequency resource (for different UEs). That is, channels are obtained by Code Division Multiplexing (CDM) of sequences if they are transmitted in the same resource block. Since only one sequence is selected and transmitted for one UE, channel selection preserves the single-carrier property of the signal. For LTE Rel-10, channel selection is used in the context of conveying ACK/NACKs from multiple component carriers, or serving cells. This applies for up to 4 HARQ-ACK bits and is defined for both FDD and TDD with aggregation of up to two serving cells. To encode the ACK/NACK/DTX information, a mapping is used between the different states of ACK, NACK and DTX, and the channels and QPSK constellation points.
In LTE Rel-10 there is no transmit diversity defined for PUCCH format 1b with channel selection. However, a few transmit diversity schemes have been given in prior art. One such scheme is Spatial Orthogonal Resource Transmit Diversity (SORTD), which uses two mutually disjoint sets of channels on the different antenna ports. The same QPSK symbol is transmitted on both antenna ports. This gives good performance but the drawback of this scheme is that the number of channels that need to be reserved for a UE doubles compared to not using any transmit diversity scheme.
To mitigate this channel overhead issue, another class of schemes has been given in prior art which are referred to as Enhanced Spatial Orthogonal Resource Transmit Diversity (E-SORTD). For these schemes the same set of channels can be used on both antenna ports. Another scheme in prior art is Space-Code Block Coding (SCBC), where in addition to use the same set of channels on both antenna ports, the modulation symbols on the different antenna ports are determined by Alamouti encoding.
According to 3GPP TS36.211 (Rel-10), the PUCCH format 1b in a slot comprises 7 OFDM symbols (6 if extended cyclic prefix is configured) of which 4 OFDM symbols contain the QPSK modulated data sequence and 3 OFDM symbols (2 if extended cyclic prefix is configured) contain a demodulation reference signal sequence. The demodulation reference signal serves as a phase reference for the detection of the QPSK symbol and the data sequence. The sequences are constructed from a QPSK sequence which is further modulated with a complex exponential function (generating a cyclic shift in the time domain) and an orthogonal cover code. The resource on antenna port {tilde over (p)} used for transmission of the PUCCH format 1b is identified by a resource ηPUCCH(1,{tilde over (p)}). This resource is also referred to as the channel. The resource value is used in deducing the details of the sequence and the frequency resources to be used for transmitting the sequence, e.g. cyclic shifts, orthogonal cover code and assigned resource block. In LTE Rel-10, the demodulation reference sequence is related to the data sequence in a predefined manner i.e. both are deduced from the same resource value. Hence, the signal space that can be used for transmitting the HARQ-ACK information comprises the selection of the resource and the QPSK symbol. In Sec. 10.1 of 3GPP TS36.213, tables are contained mapping the information states ACK, NACK and DTX to uplink control channel resources and QPSK symbols.
In E-SORTD, the signal space is extended by relaxing the assumption on a pre-determined relation between the data and reference signal sequence. When the reference signal sequence can be selected independently, the signal space comprises the resource for the data sequence, the resource for the reference signal sequence and the QPSK symbol.